This invention relates to a method of developing an electrostatic charge pattern formed on the surface of an image carrier member and, more particularly, to a method of developing such a pattern based on a magnetic brush method by using a developer composed of a magnetic carrier and a magnetic toner.
Ordinarily, a magnetic brush method is used for development of electrostatic charge patterns formed on image carrier surfaces, and binary developers in which a magnetic carrier and a non-magnetic toner are mixed are generally used as toners for magnetic brush development. A method of using a one-component developer having a resin and a magnetic powder as a developer is also known. The former method of using a binary developer ensures formation of images improved in image density and resolution but entails the problem of the image being inferior in reproduction of halftones. The method of using a one-component developer entails the problem of the occurrence of toner charging agglomeration with an increase in the toner charge and the problem of underdevelopment due to deficiency of toner on the sleeve surface serving as a developer supplying means. A means for solving these problems has been proposed which is based on using a type of developer such as those disclosed in Japanese Patent Unexamined Publication No. 59-162563 and U.S. Pat. No. 4,640,880. This type of developer is recognized to have both the advantages corresponding to those of the above two types of conventional developers, i.e., the binary developer and the one-component developer.
Although the method of developing an electrostatic charge pattern by using a developer consisting of a mixture of a magnetic carrier and a magnetic toner has both the advantages corresponding to those of the methods of using a binary developer and the method of using a one-component developer, it entails the problem cf a reduction in the definition of the pattern if the coercive force of the magnetic toner or the magnetic powder content in the magnetic toner is small. This is because the developer carrying performance of the non-magnetic sleeve incorporating a magnetic field producing member such as a permanent magnet and disposed so as to face the image carrier surface (or photosensitive body) is so inferior that the supply of the magnetic toner for the consumption owing to the friction between the magnetic brush and the image carrier surface is inadequate and that the magnetic carrier density is therefore increased. It is undesirable to simply increase the rotational speed of the sleeve or the magnetic field producing member serving as the developer supplying means in order to solve this problem, because increasing the rotational of this member produces noise and considerably deteriorates the environment. Also, increasing the outside diameter of the sleeve or the magnetic field producing member is not acceptable because it is contrary to recent demands for development apparatus reduced in size, thickness or weight.
To solve these problems, the applicant of the present invention has applied for a patent relating to the composition of a magnetic toner in which the coercive force is 50 to 600 Oe while the magnetic powder content is within a range of 10 to 75% by weight, and in which the coercive force is not smaller than 150 Oe when the magnetic powder content is less than 50% by weight, or is not larger than 250 Oe when the magnetic powder content is equal to or larger than 50% by weight (refer to Japanese Patent Unexamined Publication No. 62-275280).
This improvement invention solves the problems of the conventional art and, hence, formation of sharp images with improved transfer and fixation performance based on using a electrostatic charge pattern development method in which a magnetic carrier and a magnetic toner are used. However, it has been proved that this improvement invention does not eliminate of the possibility of occurrence of a tailing phenomenon, i.e., a phenomenon in which a slight amount of magnetic toner is attached to the rear end of the image pattern. This phenomenon is considerable particularly in the case of a solid-black image (in which the area of the black portion is excessively increased). A further improvement in this kind of development method is therefore required.